latter possibly being fiamme that are strongly flattened. The thickness of these lenses is between
1 and 10 mm. Both the light and dark bands have visible phenocrysts of quartz and feldspar, al-
though the percentage of phenocrysts is lower in the darker ones than the lighter ones.
Also present are occasional lithic fragments of mainly basement granite and irregular, though
elongated isotropic bodies that may represent remnant fiamme in the rhyolitic tuff. Similar fea-
tures in rocks from the Ritchie area, which are presently stratigraphically correlated with the
Makwassie Formation, were described as fiamme in eutaxitic ignimbrites Potgieter and Lock,
1978. The bulk of the Makwassie Formation, however, consists of a unit of massive quartz-
feldspar porphyry, with phenocrysts of quartz, feldspar and cryptocrystalline material. The latter
is probably a devitrification product of volcanic glass, visible as distinct patches of differing grain
size. Apatite, zircon, sphene and opaque minerals are present in accessory amounts, whereas sericite
flakes and chlorite are present as alteration prod- ucts. The feldspar phenocrysts in the clasts of the
basal unit show a high degree of alteration, whereas higher up in the Makwassie succession,
they are in a better state of preservation. The contact between the basal unit with the clasts and
the banded unit is sharp, whilst the latter unit grades into the upper massive rhyolite Fig. 9.
4. Structures in the basal unit
The round structures in the basal unit Fig. 7 vary in size from a few millimeters up to close to
500 mm. Most of the larger structures have cavi- ties of different shapes and sizes in the centre,
similar to the lithophysae described for the Blue Creek tuff, Snake River Plate 3.7, McPhie et al.,
1993. Some have cavities of a septarium nature Grobler et al., 1989, whilst others have cavities
comparable to a bow-tie shape, stellar, triangular and irregular. The quartz-feldspar porphyry be-
tween the structures is invariably highly altered, probably as a result of the interaction with steam
Fig. 9. Schematic profile depicting the contact between the Kameeldoorns Formation and the lower parat of the Makwassie Formation at T’Kuip Hills.
Fig. 10. Different size clasts-note the cavities in the larger ones. Scale object is 15 mm long.
from the underlying sediments during deposition Figs. 10 and 11. The material between the clasts
seems to be non-welded, but its altered nature precludes a definite conclusion. Some examples of
these structures are presented in Figs. 10 – 13.
Interesting features were observed in two struc- tures that were cut in half Figs. 14 and 15. In
both instances the samples have an outer shell of quartz-feldspar porphyry, 15 Fig. 14 and 10 mm
thick Fig. 15, respectively. Inward this is fol- lowed by a quartz sphere with a maximum thick-
ness of 20 mm in Fig. 15 and 3 mm in Fig. 14. Alternating concentric spheres of quartz-feldspar
porphyry and quartz inwardly follows this zone of quartz. The quartz infilling probably took place
by hydrothermal action soon after emplacement.
Fig. 11. Similar to Fig. 10 but in this case on the contact with underlying sediment. No major disruption of the sedimentary
material occurs at this locality. Fig. 13. Stellar-shaped cavity in a spherical clast. Note smaller
clasts above the large one.
Fig. 14. Example of concentric quartz-feldspar shells with quartz between the shells and a quartz-filled cavity in the
centre. Sample is 150 mm long. Fig. 12. Cavities in clasts — in this case some of the cavities
are lined with quartz. Cases were noted where the cavities were completely filled by quartz.
Fig. 15. Another example similar to Fig. 14 but with a thick outer cavity filled by quartz. Sample is 100 mm from left to
right.
and crowded with large siliceous nodules up to 400 mm in diameter. Smaller nodules of up to 50
mm are also mentioned. Wright and Coward 1977 regarded the pres-
ence of such nodules as a good indication of deposition of an ash-flow into water or onto
water saturated sediments. Reedman et al. 1987 described a similar setting for the Pitts Head Tuff
Formation to that of the Makwassie Formation at T’Kuip where the tuff was deposited upon
alluvial fans and fan fringe sediments. They as- cribe the growth of the siliceous nodules to the
entrapment of volatiles near the base of the zone of most intense welding.
Different possible scenarios can thus be envis- aged for the development of these structures at
T’Kuip. The flattened base of some of these fea- tures may be indicative of a very high temperature
or alternatively secondary heat produced by the overlying pile of hot ash in the process of being
welded. This scenario is, however, regarded as unlikely as the sediments underneath would have
exerted a cooling effect near the base of the unit. Another possibility is fast cooling in a gas-rich
environment, whilst another can be a process of accretion in a high temperature ash-flow in the
presence of water vapour. In an experiment where granitic material was melted with a flux and
quenched in water, a structure similar to those described in the basal unit was produced. The
structures in the basal unit were only observed at this one locality and even though many bore hole
cores penetrate the MakwassieGoedgenoeg con- tact with the Kameeldoorns Formation, nothing
similar was detected.
Wright and Coward 1977 have suggested that the nodules in the Pitts Head Tuff Formation
may represent original gas cavities, lithophysae, which were later filled in by silica. At the T’Kuip
occurrence, the spheres definitely do not represent gas cavities as they consist of quartz-feldspar por-
phyry. Rapid cooling may have produced volcanic glass that devitrified and produced these struc-
tures, or they may be the result of contraction as a result of cooling in the presence of volatiles such
as steam. Structures such as those in Fig. 11 seem to be primary and will probably not conform to
An indication for the cessation of hydrothermal activity is evidenced by the presence of both filled
and unfilled cavities. In the sample depicted in Fig. 15, it is evident that the spheres all rest or
converge on what probably represented the base or floor of the structure. The spheres have a
flattened appearance in this area, while they also appear
similar to
the shells
produced by
exfoliation.
5. Discussion
